1. Field of the Invention
The present invention relates to coating compositions prepared from an advanced epoxy resin composition which has been prepared from a reaction mixture comprising catechol and cycloaliphatic diglycidyl ether compounds. The coating compositions are useful, for example, as internal and external protective coating compositions for cans and other metal food and beverage packaging coatings.
2. Description of Background and Related Art
Epoxy resins are widely used in coating compositions of the interior and exterior surfaces of food and beverage containers because they provide a unique property combination of excellent resistance to chemicals, reasonable flexibility, resistance to hydrolysis, blush, corrosive food and beverages, having good thermal stability as well as inertness to taste or odor changes. A variety of manufacturing processes are used to apply such coatings to substrates. Flexibility and adhesion are essential because the coating should remain intact during the can formation process when the coated flat metal sheet is drawn into the form of the can. However, the bisphenol A based high molecular weight epoxy resins commonly used in can coating application have limited flexibility and toughness at room temperature. The elongation to break of bisphenol A based epoxy resins at room temperature is known to be poor. The toughness deficiency is an issue in certain applications, for example pre-coat post-form applications. Higher temperature is used in some can formation processing to compensate for the relatively low flexibility of the incumbent epoxy resin coatings.
Retort resistance is another beneficial property for can coating compositions. When the cans are filled with food, the contents are usually sterilized by heating the sealed can to temperatures of around 120-130° C. for about 1 to 2 hours, depending on the nature of the food contents. The coating is then in direct contact with the food contents for a considerable period of time, for example many years. During the sterilization and subsequent storage, the coating maintains its integrity so as to prevent corrosion of the metal can and to prevent metal migration and migration from fragmented species of the coatings into the can contents. Trends in the industry for improved flexibility in coatings without compromising retort resistance of coatings is open for alternative coating compositions meeting these technical challenges.